Weird rocker arm and engine power cutting out

[MaxmillianS]

I've got the stock 4.1 inline 6 and the power is cutting out around 55 mph and then it starts jerking hard (feels like a bucking horse) and if dont push the clutch in to stop it it just dies. It sometimes happens at lower speeds and if it hit the gas nothing happens and if i coast for a minute then when i hit the gas pedal the engine revs. Think its cam? the spark plug has wear on it like its getting combustion and spark. Any ideas?

[Captkaos]

Is that a new rocker?  If not is it oiling?